Interactions of some nitrogen, oxygen, and sulphur heterocycles with gadolinium and nickel chelates. A carbon-13 nuclear magnetic resonance spin–lattice relaxation study

Abstract

Electron–nuclear spin–lattice relaxation times for ¹³C in natural abundance are determined for six saturated and aromatic heterocyclic compounds containing nitrogen, oxygen, and sulphur in the presence of the metal chelates [Gd(tmhd)₃] and [Ni(tmhd)₂](tmhd = 2,2,6,6-tetramethylheptane-3,5-dionate). The measurements provide evidence of complex formation between [Gd(tmhd)₃] and pyrrolidine and tetrahydrofuran, and between [Ni-(tmhd)₂] and pyrrolidine, tetrahydrofuran, and tetrahydrothiophen. The relative affinities of the metal chelates for different ligands have been studied in a series of competition experiments, yielding the affinity order N > O > S for [Gd(tmhd)₃] and N > O ≈ S for [Ni(tmhd)₂]. For the case of [Ni(tmhd)₂] complexes it is found that the relaxation rates deviate largely from the predictions of the Solomon–Bloembergen equation. Comparisons with contact shifts are employed in order to rationalize these deviations.